1. Field of the Invention
The present invention relates to a hydrostatic transmission (hereinafter, xe2x80x9cHSTxe2x80x9d).
To be detailed, it relates to a technique for preventing a hydraulic circuit of the HST from overheating.
2. Related Art
Conventionally, there is a well-known HST having its neutral position expanded.
For example, there is a conventional transaxle apparatus equipped with a power transmission system including a conventional HST 8 for enabling a vehicle to travel forwardly and backwardly, as shown in FIG. 1. HST 8 comprises a variable displacement hydraulic pump 11 including a movable swash plate 13 and a fixed displacement hydraulic motor 21. Hydraulic pump 11 and motor 21 are fluidly connected with each other through a closed circuit comprising a first oil passage 93 and a second oil passage 94. Second oil passage 94 is hydraulically pressurized higher than first oil passage 93 during backward traveling. A charge oil passage 82 is provided for supplying the closed circuit with oil. Between charge oil passage 82 and each of first and second oil passages 93 and 94 is interposed a check valve 83. Check valve 83 provided for second oil passage 94 is bypassed by a drain oil passage including an orifice 116. Even if hydraulic pump 11 is inaccurately located in neutral, that is, swash plate 13 is slightly moved from its proper neutral position into its backward traveling range, increased pressure oil is drained from second oil passage 94 through orifice 116 so as to keep the hydraulic pressure in second oil passage 94 at its neutral level, thereby preventing a vehicle from jerking caused by unexpected driving of hydraulic motor 21.
Thus, movable swash plate 13 is provided in its movable range with a dead zone expanded from its proper neutral position into its backward traveling range for keeping hydraulic motor 21 neutral. For attaching the transaxle apparatus having HST 8 shown in FIG. 1 onto a vehicle, swash plate 13 is only required for sufficiently interlocking with a manual speed control tool such as a lever or a pedal to be positioned within the dead zone while the speed control tool is set at its neutral position, thereby easing an adjustment job before the releasing of the vehicle equipped with the transaxle apparatus.
However, actually, the operating oil drained from second oil passage 94 through the drain oil passage is not returned directly into an oil sump 9 but is returned into charge oil passage 82. Hence, the heated oil drained from second oil passage 94 is absorbed through check valve 83 into hydraulically depressed first oil passage 93 while being insufficiently cooled, thereby increasing the temperature of operating oil circulated between first and second oil passages 93 and 94 and reducing the displacement efficiency of HST 8.
An HST concerning the present invention is essentially constructed as follows: a housing, whose interior space serves as a fluid sump, contains a pair of first and second fluid passages, a hydraulic pump and a hydraulic motor. The first and second fluid passages are interposed between the hydraulic pump and the hydraulic motor so as to constitute a closed fluid circuit of the HST. A charge fluid passage is disposed in the housing so as to be connected with each of the first and second fluid passages for supplying fluid from the fluid sump into the closed fluid circuit. A drain fluid passage including an orifice is disposed in the housing so as to be connected with at least one of the first and second fluid passages so that, when hydraulic pressure in the at least one of the first and second fluid passages is increased beyond its neutral level, excessive fluid in the at least one of the first and second fluid passages is drained through the drain fluid passage to the fluid sump, thereby expanding the neutral zone of the HST for preventing a vehicle employing the transaxle apparatus from jerking while the HST is set in the expanded neutral zone.
Additionally, a center section forming the first and second fluid passages therein is disposed in the housing. In this case, the charge fluid passage may be formed within the center section.
Also, an oil filter may be disposed in the fluid sump for filtering fluid to be introduced into the charge fluid passage.
Also, a first check valve, which allows only a flow of fluid from the charge fluid passage to the first fluid passage, may be interposed between the charge fluid passage and the first fluid passage, and a second check valve, which allows only a flow of fluid from the charge fluid passage to the second fluid passage, may be interposed between the charge fluid passage and the second fluid passage.
A main object of the present invention is to provide such an HST improved in its draining system so that oil drained from the drain oil passage is not easily introduced into the charge fluid passage, thereby preventing the permeation of the insufficiently cooled fluid drained from the at least one of the first and second fluid passages into the closed fluid circuit so as to prevent the closed fluid circuit from overheating.
To attain the main object, the charge fluid passage and the drain fluid passage are open to the fluid sump while the charge fluid passage and the drain fluid passage being separated from each other. Therefore, the fluid drained from the at least one of first and second fluid passages through the drain oil passage is not directly introduced into the charge fluid passage but is introduced into the fluid sump. Thus, the drained fluid which has been heated by the HST in action is sufficiently cooled in the fluid sump before it is introduced into the charge fluid passage through the fluid sump, thereby keeping fine voluminal efficiency of the HST during its operation and enhancing the endurance of the HST.
For increasing the distance between both the openings of the charge fluid passage and the drain fluid passage in communication with the fluid sump so as to make the drained fluid further effectively cooled, if the center section is provided, the center section is made to have a first side end and a second side end opposite to each other. An opening of the charge fluid passage in communication with the fluid sump is disposed toward the first side end of the center section, and an opening of the drain fluid passage in communication with the fluid sump is disposed toward the second side end of the center section.
The opening of the charge fluid passage in communication with the fluid sump may be disposed adjacent to the first side end of the center section. Also, the charge fluid passage may be formed within the center section so as to be disposed adjacent to the first side end.
The opening of the drain fluid passage in communication with the fluid sump may be disposed adjacent to the second side end of the center section.
If the opening of the charge fluid passage in communication with the fluid sump is disposed adjacent to the first side end of the center section and the opening of the drain fluid passage in communication with the fluid sump is disposed adjacent to the second side end of the center section, the distance between both the openings can be increased to the limit.
If the oil filter for filtering fluid introduced into the charge fluid passage is provided, the charge fluid passage is open into communication with the fluid sump inside the oil filter.
In this case, the drain fluid passage may be open into communication with the fluid sump outside the oil filter so as to increase the distance between both the openings of the charge fluid passage and the drain fluid passage. Also, the opening of the drain fluid passage may be oriented oppositely to the oil filter. If the drain fluid passage allows the backflow of fluid from the fluid sump into said at least one of said first and second fluid passages, the drain fluid passage may be provided with another oil filter between the orifice and the fluid sump. Therefore, the orifice is prevented from being choked by obstacles in the fluid sump. Also, the closed fluid circuit is prevented from being damaged by the obstacles, thereby enhancing the endurance of the HST.
Alternatively, the drain fluid passage may also be open into communication with the fluid sump inside the oil filter. If the drain fluid passage allows the backflow of fluid from the fluid sump into said at least one of said first and second fluid passages, the oil filter for the charge fluid passage can also be used for filtering fluid to be introduced into the drain fluid passage, thereby reducing the number of parts and the manufacturing costs.
A second object of the present invention is to provide the HST provided with the above-mentioned improved draining system, which is further improved so as to prevent excessive fluid draining therethrough, thereby keeping the nice voluminal efficiency of the closed fluid circuit during the HST in action.
To attain the second object, a relief valve is intermediately provided in the drain fluid passage. The relief valve is closed when the hydraulic pressure in the at least one of the first and second fluid passages is increased beyond a predetermined degree. Due to the relief valve, when the HST is operated beyond the limit of the expanded neutral zone so as to increase the hydraulic pressure in the at least one of the first and second fluid passages in connection with the drain fluid passage beyond the predetermined degree, fluid is not drained from the closed fluid circuit of the HST, thereby keeping the voluminal efficiency of the HST so as to keep its nice reaction against a speed controlling operation.
A third object of the present invention is to provide the HST provided with the above-mentioned improved draining system, which is further improved so as to prevent fluid from back-flowing from the fluid sump into at least one of the first and second fluid passage in connection through the drain fluid passage, thereby preventing the permeation of obstacles in the fluid sump into the orifice and the closed fluid circuit.
To attain the third object, a third check valve is intermediately provided in the drain fluid passage. The third check valve allows only a flow of fluid from the at least one of the first and second fluid passages to the fluid sump. Even if the at least one of the first and second fluid passages is hydraulically depressed, the third check valve is closed so as to prevent fluid in the fluid sump from being introduced into the at least one of the first and second fluid passages, thereby preventing the closed fluid circuit from being damaged by the obstacles.
The third check valve is preferably interposed between the orifice and the fluid sump, so as to prevent the orifice from being choked by the obstacles in the fluid sump.
The closed fluid circuit is supplied with only fluid through either the first or second check valve while the fluid being cleaned by the oil filter.
Other and further objects, features and advantages of the invention will appear more fully from the following description.